1. Field of the Invention
The present invention relates to an amplitude compressing/expanding circuit which is suitable for use in signal processing circuits of electronic appliances such as a compact disc (CD) player or a tape recorder.
2. Prior Art
In digital audio signal reproducing apparatus such as CD players and digital audio tape recorders (DAT), an audio signal is digitized and recorded on a recording medium, then reproduced and demodulated into a corresponding analog signal, whereby, a high-quality audio signal with little noise over a wide dynamic range is obtained.
An audio signal obtained from such digital audio signal reproducing apparatus, however, has an undesirably wide dynamic range.
Because of the unnecessarily wide dynamic range, the signal waveform of this audio signal, when recorded by an analog tape recorder, is distorted at its higher signal level portion or has a deteriorated SN (signal-to-noise) ratio at its lower signal level portion.
Further, when a CD player or the like plays back such a signal in an environment where there is background noise, such as within an automobile, there arises another problem in which the signal portion at the lower signal level is lost in the background noise and becomes inaudible, or, conversely, the signal portion at the higher signal level produces an excessively large volume of sound.
To solve such conventional problems, one of the better solutions is to reduce the dynamic range of the audio signal obtained from the digital audio signal reproducing apparatus.
As shown in FIG. 1 for an input signal derived from an audio signal converted into an analog signal, the input signal at levels lower than -30 dB of the peak level is transformed to an output signal having a signal level which varies in proportion to the variation in the input signal level, the constant of proportion being 1.
For the range higher than -30 dB, the input signal also may be transformed to an output signal having a signal level which varies in proportion to the variation in the input signal level, with, however, a constant of proportion, for example, of 1/2.
By so doing, an input-output characteristic having ratios of amplitude compression (CR) of 1 at levels below the -30 db point and 2 at levels above the -30 db point is obtained (hereinafter such point, -30 db in the example, will be referred to as a threshold point, or level). As a result the dynamic range of the audio signal as a whole can be made narrower.
A prior art method using an amplitude compressing/expanding circuit 1 of the structure shown in FIG. 2 achieves such an amplitude compressing and expanding effect.
An input signal S.sub.I is input to an amplitude modulation circuit 3 through a delay circuit 2 and to a control signal generator circuit 4.
The control signal generator circuit 4 detects the signal level of the input signal and, based upon the result of the detection, outputs to the amplitude modulation circuit 3 a control signal S.sub.G having a signal level which changes in accordance with the signal level of the input signal S.sub.I.
The amplitude modulation circuit 3 is constructed of a multiplier circuit, such as a VCA (voltage controlled amplifier) or the like. By having the input signal S.sub.I amplitude-modulated by the control signal S.sub.G, an output S.sub.O is obtained having a signal level corresponding to the input signal S.sub.I, but changed according to the signal level of the control signal S.sub.G.
The delay circuit 2 is provided so that no overshoot may occur in the output signal S.sub.O.
Representing the amplitude of the input signal S.sub.I by "x" and the amplitude of the output signal So by "y", y can be expressed as a function of x and the amplitude compression ratio CR: EQU y=x.sup.1/CR ... (1)
For an amplitude compression ratio CR of 1 as when below the threshold point, the amplitude y is expressed as: ##EQU1##
The amplitude compression ratio CR of value 1 in the region below the threshold point can be obtained by generating control signal S.sub.G so that the gain "g" in the amplitude modulation circuit 3 equals 1, as expressed by EQU g=1. (3)
In contrast thereto, to obtain an amplitude compression ratio CR of value 2 as desired for when applied to the amplitude x of the input signal S.sub.I in the range above the threshold point, the amplitude "y" of the output signal S.sub.O and the gain "g" in the amplitude modulation circuit 3 are given by ##EQU2##
Therefore, an input/output characteristic providing the amplitude compression ratio CR having value of 2 for the range above the threshold point can be obtained by outputting the control signal S.sub.G so that the gain g in the amplitude modulation circuit 3 may become x.sup.-178 .
To provide such a characteristic, the control signal generator circuit 4 may be structured, for example, as shown in FIG. 3. As illustrated the input signal S.sub.I is supplied through an absolute value circuit 5 to an envelope detector circuit 6 to produce a detection signal S.sub.L proportional to the signal level of the input signal S.sub.I. This signal is then subjected to logarithmic conversion in an logarithmic converter circuit 7 and output to an adder circuit 9.
The adder circuit 9 sums the logarithmically-converted detection signal S.sub.L and a threshold point signal S.sub.H. A clipping circuit 8 receives the output of the adder circuit 9 and clips the summation signal below a 0 value and outputs the clipped signal S.sub.CL to a multiplier circuit 10.
Thus, because the input signal S.sub.I is determined by the threshold point signal S.sub.H, a clipped signal S.sub.CL is obtained which varies with respect to the input signal S.sub.I by setting the threshold point signal S.sub.H to a predetermined value.
The multiplier circuit 10 receives both the clipped signal S.sub.CL and a compression ratio controlling signal Sp and outputs a product signal thereof to the amplitude modulation circuit 3 through an exponential converter circuit 11.
By expressing the amplitude of the input signal S.sub.I at the threshold point (-30 dB in the present case) as "Y" and the amplitude of the output signal from the logarithmic converter circuit 7 as H when the input signal S.sub.I is at its peak level, the threshold point signal S.sub.H is expressed as EQU S.sub.H= H-Y/20 log.sub.e 10 .... (6)
Accordingly, a clipped signal S.sub.CL is obtained having a signal level which varies with the signal level of the input signal S.sub.I only when the input signal S.sub.I is larger than the signal level at the threshold point.
As a result, in the range below the threshold point, the clipped signal S.sub.CL clipped at the 0 value through the clipper circuit 8 is obtained, and a control signal S.sub.G is output through the exponential converter circuit 11 having a value of 1 corresponding to the 0 value.
Consequently, the gain of the multiplier circuit 3 is limited to 1 and an output signal S.sub.O is obtained having an amplitude compression ratio CR equal to 1 as shown in equation (2). Thus, the input/output characteristic for producing an amplitude compression ratio CR of 1 is obtained.
For the range above the threshold point, the input/output characteristic for producing an amplitude compression ratio of 2 will be satisfied if the gain of the multiplier circuit 3 is arranged to become h.sup.-1/2, as indicated in equation (5).
To do so a control signal S.sub.G, expressed as EQU S.sub.G =x.sup.(1-CR)/CR .... (7)
may be output with respect to the input signal S.sub.I.
More particularly, if the compression ratio controlling signal Sp is supplied to the multiplier circuit 10 so that the gain therein is expressed as EQU Ap=-(CR-1)/CR=-1/2 .... (8)
an output signal S.sub.O can be obtained whose amplitude is compressed by the compression ratio CR (CR=2 in the present case) in the range above the threshold point.
Thus, an amplitude compressing/expanding circuit 1 can generate an input/output characteristic which changes the amplitude compression ratio CR from 1 to 2 when the signal level of the input signal S.sub.I is increased across the signal level at -30 db.
However, in the above-described conventional circuit arrangement, the amplitude compression ratio CR abruptly changes between the front range and the rear range of the threshold point, and therefore, when the audio signal is reproduced in the conventional amplitude compressing/expanding circuit I, the reproduced sound is extremely unnatural to listeners.
To solve such problem, one conventional method has been proposed in which the amplitude compression ratio CR is gently changed by gradually changing the signal levels of the threshold point signal S.sub.H and the compression ratio controlling signal Sp in accordance with a change in the signal level of the input signal by using, for example, a ROM (read only memory) table.
Another conventional method has been proposed in which the threshold point signal S.sub.H and the compression ratio controlling signal Sp are controlled by a control circuit having a processing circuit arrangement instead of ROM table.
However, if such conventional methods are used, the construction of the amplitude compressing/expanding circuit as a whole becomes complex. Therefore, such methods are not yet satisfactorily practicable in the recent technology.
Further, because the output signal S.sub.O of the above-described amplitude compressing/expanding circuit is only obtained as a multiplied output signal S.sub.O depending upon the control signal S.sub.G provided from the control signal generator circuit 4, the operating characteristic cannot be variable in accordance with the frequencies of the input signal.
The capability of making the operating characteristic of an amplitude compressing/expanding function variable with the frequencies of the input signal is widely applicable to the arts for handling audio signals. For example, the input signal is applied to the compressed amplitude audio signal to compensate for the lack of the dynamic range that can be heard.